THE NU-F-NU PEAK ENERGY-DISTRIBUTIONS OF GAMMA-RAY BURSTS OBSERVED BY BATSE

The majority of gamma-ray bursts exhibit a peak in their nu F-nu photon energy spectra at an energy E(p) that is in the range of similar to 20-2000 keV available to the Large Area Detectors of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. If gamma-ray burst sources are at cosmological distances (d greater than or similar to 1 Gpc), then the spectra of dim bursts should be redshifted to lower energies relative to those of bright bursts. The magnitude of the shift is a function of the cosmological redshifts z of both the dim and bright burst sources and hence yields the range of redshift available to the bursts; this range is further constrained by considering cosmological model fits to the burst number-intensity distribution. We have produced photon energy spectra for similar to 400 bursts using data from BATSE to investigate if this expected shift in the nu F-nu peak is observed. We find that the mean peak energies of the burst spectra are correlated with intensity: lower intensity groups of burst spectra exhibit a lower average peak energy, although the distributions of E(p) are quite wide. Denoting the redshift of an event with an observed peak photon flux P (photons cm(-2) s(-1)) by z(P), we find that the maximum range consistent with the bursts of our sample is (1 + z(1))/(1 + z(100)) = 1.86(+0.24)(+0.36).